1. Field of the Invention
This invention relates to a light valve image display apparatus of a projection type using a liquid crystal, and in particular to a liquid crystal light valve image display apparatus of a projection type which has a high projection light utilization factor and which is capable of realizing real-time display of a large color image of high quality with a single light valve.
2. Description of the Prior Art
In recent years, large size image display apparatus have been regarded as a promising product not only for the purposes of education and industry but also for large-sized home televisions. Great emphasis has been placed on the research and development of a projection type display apparatus comprising an optical projection system through which an image produced on a small screen is projected in an enlarged form on another screen. The research reports on a number of projection type image display systems and products have already been announced. Such systems in practical use include a system in which an image on a screen of a cathode ray tube is projected in an enlarged form on another screen through a lens system, and a system in which the surface of an oil film is roughened in accordance with the image by use of an electron beam so that an image is projected through a Shlieren optical system. These systems, however, have the problems of high cost and short lifetime. Another category of large-sized image projection apparatus for active research is a light valve projection system utilizing a PLZT, elastomer or liquid crystal, because of its expected high resolution and low cost. The liquid crystal, which is capable of modulating light at low voltage, is particularly interesting because it is comparatively easily driven and the device construction is simple. It is expected that the liquid crystal can realize a compact projection type image display apparatus at low cost.
Four main types of light valve using a liquid crystal have been proposed. In a first system, with an X-Y matrix arrangement of electrodes, an image is written in the liquid crystal layer with an electrical signal. A second type of system is such that an image is written by scanning an electron beam on a conductive substrate supporting the liquid crystal layer. In a third type of light valve utilizing liquid crystal, a laser beam is radiated on the liquid crystal layer and the resulting heat is used to cause local phase transition of the liquid crystal, thus writing an image. A fourth type of such a light valve comprises a photoconductive layer and a liquid crystal layer which are laid one on the other and arranged between electrode substrates, and an image is focussed on the photoconductive layer from a two-dimensional light source to supply an image signal in the liquid crystal layer by the use of the distribution of resistance variation in the photoconductive layer.
Applications of various phenomena and effects of the liquid crystal to the light valve are roughly divided into three types. They are a method utilizing light scattering phenomena such as DSM (dynamic scattering mode) and PT (phase transition), a method utilizing an electro-optic effect such as ECB (electrically controllable birefringence) or HFE (hybrid field effect), and a method utilizing light diffraction phenomena by the domain occuring in the liquid crystal such as WD (Williams Domain) or VGM (variable grating mode).
In the liquid crystal light valve projection system using light scattering, the light scattered from the liquid crystal is concentrated for image formation on a screen by use of a Schlieren optical system. Since light is scattered at wide angles, a large lens is required to condense it, and therefore it is rather difficult for this system to produce a bright image of high contrast for white-on-black display. The other disadvantages of this system include lack of sharpness of image edges, difficult display of half tones, resulting in the lack of image uniformity, and high driving voltage leading to a short lifetime.
The liquid crystal light valve projection system using the electro-optic effect has a number of advantages including a low voltage, long lifetime, high resolution, high contrast, multiple tones or gradation and quick response time. The shortcomings, however, are that in view of polarization type, the reproduced image is dark and it is not possible to produce a high-quality color display image due to an interference effect.
In the image projection system of liquid crystal light valve type using the light diffraction caused by domain in the liquid crystal layer, on the other hand, the diffracted light is projected on the screen by use of the Schlieren optical system. Since the diffracted light is clearly separated from non-diffracted light and the diffracted light does not spread widely from the optical axis, the light utilization efficiency is high on one hand and a bright and high-contrast image is produced on the other hand. Nevertheless, the Williams domain exists stably for only a small voltage range and therefore display of half tone is difficult. Further, it is low in response speed, and it is difficult to control the diffracted light in view of the fact that the domain pitch is determined by the thickness of the liquid crystal layer. Lastly, the light valve using VGM has a short lifetime due to the fact that the light valve is actuated with direct current.